The present invention relates to embodiments of an improved universal modular prosthesis stem extension. The inventive stem extension constitutes an improvement over an invention disclosed in U.S. Pat. No. 4,985,037 in the name of coinventor Thomas D. Petersen, M.D. The entirety of the above-mentioned pending United States Patent Application is hereby incorporated by reference herein, including the discussion of prior art contained therein.
In the field of orthopedics, a definite clinical need exists for improved stem extensions for both femora and tibial components. Each year, more and more failed primary total knee and hip arthoplasties exist due to loosening, wear, or other reasons. When a total knee or hip arthoplasty fails, removal of the primary components thereof is necessitated, which commonly results in considerable bone loss necessitating a stem fixation in the subsequent implant.
Clinical experience has shown that large stem extensions act to stabilize the prosthesis through transmission of surface stresses to the stem and consequently to the cortical bone, thereby stress shielding deficient bone from torsional stresses in the immediate surface bearing area. Consequently, as surgical procedures evolve, larger and longer stems are being proposed for revision surgery.
The problem is complicated because anatomical variations in different patients require a surgeon to carry in inventory four to eight sizes of femoral components and ten to fifteen sizes of tibial components. Further complexity is added by virtue of the fact that the distal femoral stem has to be at a 5-9 degree valgus angle to the femoral components to fit the intermedullary canal of the femur. Thus, these prosthetic components require a great number of fixed stems. One manufacturer, for example, has at leas& nine different femoral stem sizes ranging from 10 millimeters to 22 millimeters in diameter. Considering the fact that they also make six femoral prostheses, for each of the left and right-hand sides, it is easy to see that over one hundred sizes and shapes of fixed prostheses have to be manufactured and must be carried in inventory by the surgeon or hospital. The high expense of each prosthesis makes carrying such a large inventory of prostheses exhorbitantly expensive.
If a stem extension could be created which could fit on the undercarriage of the articular surface of any one of a multiplicity of prostheses and, further, if such a stem extension could be designed to be adjustable to any one of a multiplicity of positions on any particular prosthesis undercarriage, the above-described inventory of prostheses could be reduced even below the six prostheses and nine reversible stem extensions to which the inventory may be reduced through the teachings of the invention disclosed in copending U.S. Patent Application Ser. No. 07/354,792.
As stated in the copending application, there are anatomical considerations in the design of a stem extension which make it desirable to have such a stem extension angled in the lateral plane as well as in the medial lateral plane. Ideally, the distal femoral cut of the femur is at 90 degrees, or perpendicular to the angular bow of the femur as seen in the lateral plane. The angle of bowing of the femur may vary from 0 up to 12 degrees; however, most femurs bow from 3 to 6 degrees. Accordingly, fitting a fixed straight stem extension into the intermedullary canal when there is significant degree of bowing may be quite difficult. However, angling of the stem extension with respect to the axis of the prosthesis facilitates more effective installation of the stem within the intermedullary canal.
As pointed out in the copending application, additional complications exist when a stem extension is included in a tibial component. As is known to those skilled in the art, some instrumentation systems which are designed to measure the exact location of tibial cuts result in cutting of the top of the tibia at varying angles such as, for example, 10 degrees, 7 degrees, 3 degrees, and some absolutely perpendicular. Thus, a properly designed stem extension could be used to modify a tibial component to the particular method of instrumentation which is utilized at minimal cost. While most surgeons use a 3-degree sloping cut on the proximal tibia, the surgical group of co-applicant Petersen has been using a 10-degree slope for a number of years and finds this slope to be advantageous in obtaining more flexion and in controlling the forward movement of the femur on the tibia when the patient is descending stairs and making other similar movements. In such a situation, of course, the prosthesis could be initially installed without the stem extension and, if a refit is necessitated, the stem extension could be easily installed.
The invention disclosed in the U.S. Pat. No. 4,985,037 constitutes a vast improvement over the prior art known at the time of the invention thereof. The invention disclosed therein contemplates providing each prosthesis which is to incorporate the inventive stem extension with an undersurface having two parallel flat walls extending upwardly from the undersurface of the prosthesis to form a generally rectangular cubic chamber therebetween. Pairs of aligned openings are formed in these walls and are designed to receive mounting pins which may also extend through holes and/or slots formed in an attachment means forming a portion of the inventive stem extension.
The attachment means of the stem extension disclosed in the copending application includes two corresponding flat parallel walls designed to slidably engage the internal surfaces of the parallel walls of the undersurface of the particular prosthesis. With this construction, this stem extension may be installed on a particular prosthesis in any one of two orientations, which orientations are rotated 180 degrees with respect to one another.
In the preferred embodiment of the stem extension disclosed in the co-pending patent application, each stem extension has a 6-degree valgus inclination as well as a degree of superior or inferior angulation such as, for example, 2 degrees. This inclination and angulation are anatomically correct in each reversed position of the modular stem extension. More particularly, the position of the stem extension, as properly mounted on a left-hand prosthesis, is anatomically correct, as is the position of the stem extension as properly mounted on a right-hand prosthesis.
While the invention disclosed in the U.S. patent is far superior to any prior designs, it is limited to only two positions of the stem extension for each prosthesis. As is known, anatomical variations of the intermedullary canals of various patients have created a need for increased versatility in prosthetic stem extensions, even over and above the versatility provided by the invention disclosed in the U.S. patent. The coupling mechanism between the stem extension and the associated undersurface of the associated prosthetic head is designed to limit the orientations of the stem extension with respect to the prosthetic undersurface to two variations spaced 180 degrees apart. If this coupling mechanism could be redesigned to provide more than two variations in orientation between the stem extension and the undersurface of the associated prosthetic head, vastly increased versatility and adaptability to differing conditions could be achieved. Also, inventories could be further reduced. It is with these thoughts in mind that the present invention was developed.
The following prior art is known to applicant:
U.S. Pat. No. 3,102,536 to Rose, et al. discloses a hip prosthesis wherein the head thereof is coupled to a neck portion of the associated stem assembly through the use of a wedging mechanism which may be loosened to allow removal of the head. The present invention differs from the teachings of Rose, et al. as allowing alterations in the orientation of a stem with respect to its associated head structure. This is not possible in the Rose, et al. device.
U.S. Pat. No. 4,687,486 to Brinckmann, et al. discloses an implant including a head coupled to a neck assembly. The device includes a nut 7 having recesses 16 designed to be engageable by a tongue 17 of a tool 15. The nut 7 is provided to allow release of separable parts of the prosthesis from one another. This is different from the teachings of the present invention, wherein, in one embodiment thereof, a nut having recesses about its periphery is utilized to tighten two components of a prosthesis in a desired orientation with respect to one another.
U.S. Pat. No. 4,693,724 to Rhenter, et al. discloses a total hip prosthesis having a stem attachable to a neck through the use of a wedged coupling. The present invention differs from the teachings of Rhenter, et al. as including structure allowing adjustment of the orientation of the stem portion thereof with a valgus inclination as well as angulations in the superior and inferior directions.
German Offenlegungsschrift 2114323 to Bostrom discloses an implantable prosthesis including a rotary joint between the stems thereof as best seen in FIG. 5 thereof. The present invention differs from the teachings of this document as including the possibility, in one embodiment thereof, of a universal joint between two portions of a stem of a prosthesis to allow adjustments in valgus inclination as well as in inclinations in the superior and inferior directions.
European Patent 0 000 549 to Seiler discloses a prosthesis including a stem having one portion implantable into the intermedullary canal and another portion having a curved neck and a head attached thereto. The other portion may be installed in the first mentioned portion in a variety of orientations in light of the coupling 12, 15, 6, 14. The present invention differs from the teachings of this document as allowing adjustments in valgus inclination as well as adjustments in angulation in the superior and inferior directions.
French Patent 2,575,383 to Lecestre, et al. discloses a prosthesis including a head attachable to a neck by virtue of a conical coupling including interrelated teeth which allow rotative adjustments between the two attached components. Lecestre, et al. do not teach or suggest adjustments in valgus inclination of a stem nor adjustments in angulation of a stem in the superior or inferior directions as taught herein.
European Patent 0 201 407 to Montagne discloses a femoral prosthesis wherein a single stem assembly may be attached to one of a plurality of heads by virtue of one of a multiplicity of stem extensions. The present invention differs from the teachings of Montagne as including different structure for the attachment of the various components together.